This invention pertains to measurement of contaminants or constituents in a sample, and more particularly to measurement of constituents in a sample in which the constituents are in the range of one part per billion (ppb) or less.
Measurement of constituents in a sample at levels of less than or equal to 1 ppb continues to be a challenge, particularly in the part per trillion (ppt) range. For example, ion chromatography can be used to measure anions and cations in a sample, such as in a sample of water. However, current technology for ion chromatography detectors limit sample measurement for ions to about 50 ppb or greater. Higher purity measurements are attainable with preconcentration, but the validity of the results can be affected by the calibration techniques used.
Prior art systems utilize suppression to reduce the background level of conductivity at the detector and preconcentration to collect a large volume of sample. Both techniques enhance response, and in combination, are used to achieve the lowest levels of detection.
Calibration of these systems for measurement at ppt levels is achieved by several methods. One method involves the injection of a standard much more concentrated than the samples of interest, and subsequent extrapolation of the calibration curves down to ppt levels. This process may introduce an error in the calibration, and small errors in the calibration produce large errors at the ppt level. In another method of calibration, a small volume of concentrated standard is injected into the sample stream, eliminating errors caused by extrapolation. However, this method may be susceptible to errors in measurement of the injected volume and the diluent volume, and an error in the volume or flow may cause an error in calibration.
Current requirements for advanced combustion turbines allow the use of steam to provide cooling for a turbine transition section rather than air, which was previously used. Steam has a significantly higher heat transfer coefficient than air, and thus is more efficient in cooling. In order to take advantage of the higher heat transfer coefficient, it is necessary that the steam be pure, which requires measurement of ions, typically anions, at levels less than 1 ppb, or in the ppt range.
Ion chromatography is typically used for ion measurement. U.S. Pat. Nos. 5,908,556; 5,597,481; 4,981,804; and 4,766,550 exemplify the prior art for ion chromatography; these patents are incorporated herein by reference in their entirety for all purposes. In power plants, demineralized water is used for making steam. Steam purity is obtained by consolidation of the contaminants in the steam drum and subsequently controlled by blowdown. An on-line sampling system and an ion chromatography system are preferred for frequent analysis of the steam that is used for cooling the turbine transition section. An on-line ion chromatography system for continuous, automated operation is commercially available, but successful validation of the results can be problematic at low ppt levels.
There remains a need in the art for an on-line chromatography system for continuous automated operation which can accurately measure ions present in concentrations less than 1 ppb.
This invention relates to an analytical apparatus for measuring a low concentration of a constituent in a stream at a parts per billion level. The apparatus comprises: (a) a sample loop having a predetermined volume for receiving a sample from the stream, wherein the sample loop has an inlet for receiving a sample from the stream, an outlet, and a valve for switching between sample intake and sample discharge. The apparatus further comprises: (b) a purifier for producing a substantially pure carrier liquid from a sample of the stream, (c) a first multi-path valve connected to the purifier and to the sample loop for alternately connecting the loop with a sample to be analyzed and a purified carrier liquid from the purifier, (d) a preconcentrator column in fluid connection with the outlet of the sample loop for absorbing and increasing the concentration of the low concentration constituent, (e) a second multi-path valve connected to the preconcentrator column for alternately connecting the column with the carrier liquid containing the constituent and with an eluent for the constituent, (f) a source of an eluent for the constituent, the source being connected to the second multi-path valve, and (g) a detector in fluid connection with the preconcentrator column for receiving and analyzing a constituent concentration in an eluent received from the preconcentrator column.
The invention also includes a method of analyzing a stream for a low concentration constituent present in the stream at a parts per billion level. The method comprises the steps of: (a) introducing a first sample from the stream into a sample loop having a predetermined volume for containing a predetermined volume of the first sample, so as to fill the loop with the first sample; (b) purifying a second sample of the stream to produce substantially pure carrier liquid; (c) and injecting the first sample into a preconcentrator column by forcing the first sample from the sample loop using the pure carrier liquid for absorbing and increasing the concentration of the low concentration constituent on the preconcentrator column. The method further comprises: (d) introducing an eluent into the preconcentrator column to transport an absorbed constituent from the preconcentrator column to a detector; and (e) analyzing the eluent for a concentration of the constituent therein.
The invention also provides a method of on-line calibration of an analytical apparatus for measuring a low concentration of a constituent in a stream at a parts per billion level, comprising the steps of: (a) introducing a first sample from a standard stream comprising a known low concentration of a known constituent into a standard loop having a predetermined volume for containing a predetermined volume of the first sample, so as to fill the loop with the first sample; (b) purifying a second sample of the standard stream to produce substantially pure carrier liquid; (c) injecting the first sample into a preconcentrator column by forcing the first sample from the standard loop using the pure carrier liquid for absorbing and increasing the concentration of the low concentration constituent on the preconcentrator column; and (d) introducing an eluent into the preconcentrator column to transport an absorbed constituent from the preconcentrator column to a detector. The method further comprises: (e) analyzing the eluent for a concentrator of the constituent therein; (f) calculating an amount of constituent to reach the detector; and (g) determining a calibration constant for the detector.
Finally, the invention relates to an on-line method for creating a calibration curve for a detector in an analytic apparatus for measuring a low concentration of a constituent in a stream at a parts per billion level. The method comprises the steps of: (a) introducing a first sample from a standard stream comprising a known low concentration of a known constituent into a standard loop having a predetermined volume for containing a predetermined volume of the first sample, so as to fill the loop with the first sample; (b) purifying a second sample of the standard stream to produce substantially pure carrier liquid; (c) injecting the first sample into a preconcentrator column by forcing the first sample from the standard loop using the pure carrier liquid for absorbing and increasing the concentration of the low concentration constituent on the preconcentrator column; (d) introducing an eluent into the preconcentrator column to transport an absorbed constituent from the preconcentrator column to a detector; and (e) analyzing the eluent for a concentrator of the constituent therein. The method further comprises: (f) calculating an amount of constituent to reach the detector and (g) determining a first calibration constant for the detector. The method next comprises (h) repeating steps (a) through (c) twice in succession; (i) repeating steps (d) through (f); and (j) calculating a second calibration constant for the detector. Finally, the method comprises the steps of: (k) repeating steps (a) through (c) three times in succession; (l) repeating steps (d) through (f); (m) calculating a third calibration constant for the detector; and (n) combining the first and the second and the third calibration constants to create a calibration curve for the detector.